1. Field of the Invention
Embodiments of the present invention relate to a unit pixel of an image sensor and a photo detector of the unit pixel.
2. Description of the Related Art
An image sensor is a sensor that transforms an optical signal to an electrical image signal. When light is irradiated to a light-absorbing part inside a unit pixel of an image sensor chip, the image sensor detects the light incident at each unit pixel and the amount of the light and transforms an optical signal to an electrical signal and then transfers the electrical signal to analog and digital circuits for forming an image.
The conventional image sensors can be classified into CCD (Charge Coupled Device) types and CMOS (Complementary Metal Oxide Semiconductor) types, based on their structures and operation principles. The CMOS type image sensors are commonly referred to as CIS (CMOS Image Sensor).
In the CCD type image sensor, groups of signal electrons generated at the pixels by the light are transmitted to an output unit by a pulse applied to a gate, transformed to voltages of the output unit, and sent out one by one.
In the CMOS type image sensor, the signal electrons and holes that are generated at the pixels by the light are transformed to voltages inside the pixels. These voltages are connected to a signal processor, including a row decoder and a column decoder, and sent out of the pixels by a switching operation according to a clock frequency.
The image sensor can be either an APS (Active Pixel Sensor) or a PPS (Passive Pixel Sensor), according to the presence of an amplifier in the unit pixel.
The PPS is a passive device that does not encompass a signal amplification function inside the pixel, and outputs the electric current of the device to the outside to transform the electric current to a voltage outside the pixel. On the other hand, the APS is an active device that encompasses a signal amplification function inside the pixel.
The PPS is mostly constituted with one photo diode and one select transistor, and thus not only can have a greater aperture ratio than the APS, which requires 3-MOS transistors for the same sized pixel, but also can raise a fill factor related to a light-absorbing efficiency.
However, since the intensity of photoelectric current of the photo diode is not great and an optical signal is transformed to electric current that is vulnerable to an outside environment for use in signal processing, the PPS has a problem of fixed pattern noise (FPN).
Therefore, for the same size pixel, the APS can provide an image signal that has relatively less noise than the PPS, despite the smaller size of the light-absorbing part than the PPS, since a multiple number of transistors are present in the unit pixel.
One electron-hole pair (EHP) is generated for one photon that is incident at a unit pixel light-absorbing part of an image sensor, and the generated electrons and holes are accumulated in a photo diode, which is a light-absorbing part.
The maximum accumulation electrostatic capacity of a photo diode is proportional to the area of photo detection of the photo diode. Particularly, in the case of CMOS type image sensor, the area in which the accompanying transistors are arranged is larger than that of the CCD type image sensor, and thus increasing the area of the light-absorbing part is physically limited. Moreover, the photo diode, which is commonly used as the light-absorbing part of an image sensor, has relatively small electrostatic capacity and thus is easily saturated, and it is difficult to segment the signals generated by the light-absorbing part.
Therefore, the unit pixels of the CMOS image sensor require a relatively long photoelectric charge accumulation time in order to generate a minimum electric charge for signal processing through the limited photo detection area. Accordingly, it is not easy to manufacture a high-density/high-speed frame image sensor by using the unit pixels having this kind of light-absorbing part.
The band gap of a silicon semiconductor is 1.12 eV, and a photo detector made of a silicon semiconductor can detect light energy in wavelengths of 350 nm to 1150 nm. Here, since the light has different inherent energy per wavelength and has different depth of penetration when the light penetrates silicon, which is solid, the photoelectric efficiency for each wavelength is also different at the photo detector. In order to detect the wavelengths of visible rays (400-700 nm), the image sensor forms an interface of P-N junction so that a green ray, which commonly has energy in the wavelength of 550 nm, can be better detected. Therefore, in the image sensor having this structure, photoelectric efficiencies for a short wavelength, such as blue color, and a long wavelength of a near infrared ray are deteriorated, or the optical signal is transformed to a noise.
Prior arts related to an image sensor and a unit pixel of an image sensor include U.S. Publication Number 2004/0217262 (“UNIT PIXEL IN CMOS IMAGE SENSOR WITH HIGH SENSITIVITY”), U.S. Publication Number 2009/0032852 (“CMOS IMAGE SENSOR”) and U.S. Publication Number 2010/0073538 (“IMAGE SENSOR”).
U.S. Publication Number 2004/0217262 discloses an image sensor that includes one photo diode and four transistors of a transfer transistor, a reset transistor, a drive transistor and a selection transistor and that inhibits the drive transistor and the selection transistor from being affected by leakage of a power supply voltage (VDD) by separating an active area in which the drive transistor and the selection transistor are formed from an active area in which the reset transistor is formed.
However, since U.S. Publication Number 2004/0217262 integrates the photo diode and the four transistors in a limited area, it is difficult to provide an area of the photo diode for generating a sufficient quantity of electric charge for signal processing.
U.S. Publication Number 2009/0032852 discloses an image sensor that can acquire a wide dynamic range without the loss of sensitivity, by allowing a pixel constituting a CMOS image sensor to have a plurality of floating diffusion regions.
The CMOS image sensor of U.S. Publication Number 2009/0032852 acquires a final image by acquiring and synthesizing a signal, of which the sensitivity is low but the dynamic range for the brightness is wide, and a signal, of which the dynamic range for the brightness is narrow but the sensitivity is high, in a separate floating diffusion region.
However, since the above CMOS image sensor acquires the high-sensitivity signal and the wide dynamic range signal using the respective separate floating diffusion regions and their related transistors, it is difficult to provide a sufficient region for a photo detector.
U.S. Publication Number 2010/0073538 discloses an image sensor having a high photoconductivity. However, the image sensor of U.S. Publication Number 2010/0073538 forms an additional film layer over a PN junction in order to increase the photoconductivity of a PN junction diode, and thus requires an additional manufacturing process.